1. Field of the Invention
The present invention relates in general to adjustment of a head switching point in a video cassette tape recorder (VCR), and more particularly to an apparatus and method for automatically controlling adjustment of a head switching point in a VCR, whereby automatic adjustment of the head switching point can be substituted for conventional manual adjustment thereof and performed during every tape playback routine to provide stable image quality.
2. Description of the Prior Art
Referring to FIG. 1, there is shown a block diagram of a conventional head switching point adjustment apparatus for a VCR. The illustrated apparatus comprises an amplifier 1 for amplifying a drum pulse signal P.G outputted from a drum pulse generator (not shown), a delay monostable multivibrator 2 being triggered by a signal output from the amplifier 1 to output a pulse signal delayed by a time constant which is determined by a variable resistor VR1 and a capacitor c1, a frequency generator 3 responsive to the pulse signal output from the delay monostable multivibrator 2 for outputting a head switching signal with a predetermined frequency of 30Hz.
The operation of the conventional head switching point adjustment apparatus for the VCR with the above-mentioned construction will hereinafter be described with reference to FIG. 2.
Upon application of the drum pulse signal P.G as shown in the first line of FIG. 2, the amplifier 1 amplifies the received drum pulse signal P.G by a predetermined amplification amount and applies the amplified pulse signal as a trigger signal to the monostable multivibrator 2. Upon being triggered by the pulse signal from the amplifier 1, the monostable multivibrator 2 outputs a pulse signal PS as shown in the second line of FIG. 2 delayed by a time constant which is determined by the variable resistor VR1 and capacitor c1.
The pulse signal from the monostable multivibrator 2 is input to the frequency generator 3. Generated in the frequency generator 3 is a frequency signal (30 Hz) toggled at a falling edge of the pulse signal from the monostable multivibrator 2, as shown in the third line of FIG. 2. The generated frequency signal (30Hz) from the frequency generator 3 is outputted as a head switching signal HSW30Hz.
It is generally prescribed that a vertical synchronous signal Vsync is delayed by 6.5H(=412 .mu.sec) behind a switching point of the head switching signal HSW30Hz. This means that the switching point of the head switching signal HSW30Hz from the frequency generator 3 as shown in the third line of FIG. 2 must be faster by 6.5H than the vertical synchronous signal Vsync as shown in the fourth line of FIG. 2.
For this reason, an interval between the switching point of the switching signal frequency HSW30Hz and the vertical synchronous signal Vsync must be adjusted to 6.5H. To achieve that adjustment, the variable resistor VR1 for the monostable multivibrator 2 must be adjusted to vary the time constant.
The adjusting process of various setting point values is usually done in a VCR manufacturing plant where a technician performs the adjustment of the switching point by adjusting the time constant variable resistor VR1 for the monostable multivibrator 2. The technician determines a desired resistance point for the variable resistor VR1 while observing waveforms of the head switching signal frequency HSW30Hz and the vertical synchronous signal Vsync displayed on an oscilloscope.
However, the conventional head switching point adjustment apparatus for the VCR has a disadvantage, in that the adjustment of the head switching point is performed manually. This manual adjustment process results in inconvenience, a high rate of error generation and the need for attendant adjusting equipment such as an oscilloscope.
Also, a VCR in which a volume of the variable resistor VR1 is altered by an external vibration and erroneously adjusted is not compatible with a different VCR. This results in a noise on the lower portion of the screen caused by the switching error resulting from using two different VCRs interchangeably. Further, in playing back a video tape recorded by the different VCR, the vertical synchronizations of the two VCRs do not match each other.